The proportion of tyrosine phosphorylated spermatozoa in cryopreserved semen is negatively related to crossbred bull fertility

Male infertility and perfluoroalkyl and poly-fluoroalkyl substances: evidence for alterations in phosphorylation of proteins and fertility-related functional attributes in bull spermatozoa†

BACKGROUND

Perfluoroalkyl and poly-fluoroalkyl substances (PFAS) are pervasive environmental pollutants and potential threats to reproductive health. Epidemiological studies have established an association between PFAS and male infertility, but the underlying mechanisms are unclear.

OBJECTIVES

Investigate the effect of perfluorooctane sulfonic acid (PFOS), the most prevalent and representative PFAS, on bull sperm protein phosphorylation and function.

METHODS

We exposed bull sperm to PFOS at 10 (average population exposure) and 100 μM (high-exposure scenario), and analyzed global proteomic and phosphoproteomic analysis by TMT labeling and Nano LC-MS/MS. We also measured sperm fertility functions by flow cytometry.

RESULTS

PFOS at 10-μM altered sperm proteins linked to spermatogenesis and chromatin condensation, while at 100 μM, PFOS affected proteins associated with motility and fertility. We detected 299 phosphopeptides from 116 proteins, with 45 exhibiting differential expression between control and PFOS groups. PFOS dysregulated phosphorylation of key proteins (ACRBP, PRKAR2A, RAB2B, SPAG8, TUBB4B, ZPBP, and C2CD6) involved in sperm capacitation, acrosome reaction, sperm-egg interaction, and fertilization. PFOS also affected phosphorylation of other proteins (AQP7, HSBP9, IL4I1, PRKAR1A, and CCT8L2) related to sperm stress resistance and cryotolerance. Notably, four proteins (PRM1, ACRBP, TSSK1B, and CFAP45) exhibited differential regulation at both proteomic and phosphoproteomic levels. Flow cytometric analysis confirmed that PFOS increased protein phosphorylation in sperm and also decreased sperm motility, viability, calcium, and mitochondrial membrane potential and increased mitochondrial ROS in a dose-dependent manner.

CONCLUSIONS

This study demonstrates that PFOS exposure negatively affects phosphorylation of proteins vital for bull sperm function and fertilization.

Paternal Age Amplifies Cryopreservation-Induced Stress in Human Spermatozoa

The global fall in male fertility is a complicated process driven by a variety of factors, including environmental exposure, lifestyle, obesity, stress, and aging. The availability of assisted reproductive technology (ART) has allowed older couples to conceive, increasing the average paternal age at first childbirth. Advanced paternal age (APA), most often considered male age ≥40, has been described to impact several aspects of male reproductive physiology. In this prospective cohort study including 200 normozoospermic patients, 105 of whom were ≤35 years (non-APA), and 95 of whom were ≥42 years (APA), we assessed the impact of paternal age on different endpoints representative of sperm quality and cryopreservation tolerance. Non-APA patients had superior fresh semen quality; DNA fragmentation was notably increased in APA as compared to non-APA individuals (21.7% vs. 15.4%). Cryopreservation further increased the DNA fragmentation index in APA (26.7%) but not in non-APA patients. Additionally, APA was associated with increased mtDNAcn in both fresh and frozen/thawed sperm, which is indicative of poorer mitochondrial quality. Cryopreservation negatively impacted acrosome integrity in both age groups, as indicated by reduced incidences of unreacted acrosome in relation to fresh counterparts in non-APA (from 71.5% to 57.7%) and APA patients (from 75% to 63%). Finally, cryopreservation significantly reduced the phosphorylation status of proteins containing tyrosine residues in sperm from young males. Therefore, the present findings shed light on the effects of paternal age and cryopreservation on sperm quality and serve as valuable new parameters to improve our understanding of the mechanisms underlying sperm developmental competence that are under threat in current ART practice.

Cryopreservation process alters the expression of genes involved in pathways associated with the fertility of bull spermatozoa

In bovines, cryopreserved semen is used for artificial insemination; however, the fertility of cryopreserved semen is far lower than that of fresh semen. Although cryopreservation alters sperm phenotypic characteristics, its effect on sperm molecular health is not thoroughly understood. The present study applied next-generation sequencing to investigate the effect of cryopreservation on the sperm transcriptomic composition of bull spermatozoa. While freshly ejaculated bull spermatozoa showed 14,280 transcripts, cryopreserved spermatozoa showed only 12,375 transcripts. Comparative analysis revealed that 241 genes were upregulated, 662 genes were downregulated, and 215 genes showed neutral expression in cryopreserved spermatozoa compared to fresh spermatozoa. Gene ontology analysis indicated that the dysregulated transcripts were involved in nucleic acid binding, transcription-specific activity, and protein kinase binding involving protein autophosphorylation, ventricular septum morphogenesis, and organ development. Moreover, the dysregulated genes in cryopreserved spermatozoa were involved in pathways associated with glycogen metabolism, MAPK signalling, embryonic organ morphogenesis, ectodermal placode formation, and regulation of protein auto-phosphorylation. These findings suggest that the cryopreservation process induced alterations in the abundance of sperm transcripts related to potential fertility-associated functions and pathways, which might partly explain the reduced fertility observed with cryopreserved bull spermatozoa.

Transcriptomic Profiling of Buffalo Spermatozoa Reveals Dysregulation of Functionally Relevant mRNAs in Low-Fertile Bulls

Although, it is known that spermatozoa harbor a variety of RNAs that may influence embryonic development, little is understood about sperm transcriptomic differences in relation to fertility, especially in buffaloes. In the present study, we compared the differences in sperm functional attributes and transcriptomic profile between high- and low-fertile buffalo bulls. Sperm membrane and acrosomal integrity were lower (P < 0.05), while protamine deficiency and lipid peroxidation were higher (P < 0.05) in low- compared to high-fertile bulls. Transcriptomic analysis using mRNA microarray technology detected a total of 51,282 transcripts in buffalo spermatozoa, of which 4,050 transcripts were differentially expressed, and 709 transcripts were found to be significantly dysregulated (P < 0.05 and fold change >1) between high- and low-fertile bulls. Majority of the dysregulated transcripts were related to binding activity, transcription, translation, and metabolic processes with primary localization in the cell nucleus, nucleoplasm, and in cytosol. Pathways related to MAPK signaling, ribosome pathway, and oxidative phosphorylation were dysregulated in low-fertile bull spermatozoa. Using bioinformatics analysis, we observed that several genes related to sperm functional attributes were significantly downregulated in low-fertile bull spermatozoa. Validation of the results of microarray analysis was carried out using real-time qPCR expression analysis of selected genes (YBX1, ORAI3, and TFAP2C). The relative expression of these genes followed the same trend in both the techniques. Collectively, this is the first study to report the transcriptomic profile of buffalo spermatozoa and to demonstrate the dysregulation of functionally relevant transcripts in low-fertile bull spermatozoa. The results of the present study open up new avenues for understanding the etiology for poor fertility in buffalo bulls and to identify fertility biomarkers.

RNA-Seq analysis reveals functionally relevant coding and non-coding RNAs in crossbred bull spermatozoa

•

RNA-Seq analysis was done to characterize the transcriptome of crossbred bull spermatozoa.

•

Among the 13,814 transcripts detected, 431 had FPKM > 1 and 13,673 had FPKM > 0 or < 1.

•

Coding and non-coding RNAs account for 13,145 (95.15%) and 152 (1.1%), respectively.

•

Sperm transcripts were mainly related to ribosome, oxidative phosphorylation and spliceosome pathways.

•

qPCR analysis showed individual variations in transcriptional abundance of selected genes.

Sperm, which are believed to be transcriptionally and translationally inactive, synthesize RNA and proteins before there is gradual disappearance of the ribosome during chromatin compaction. Sperm transfer several functionally relevant transcripts to the oocyte, controlling maternal-zygotic transition and embryonic development. The present study was undertaken to profile and analyze sperm transcripts comprehensively using Next Generation Ribonucleic acid sequencing technology in Holstein Friesian x Tharparkar crossbred bulls. The results from global transcriptomic profiling revealed transcripts for 13,814 genes; of which 431 transcripts were expressed with >1 FPKM and 13,383 transcripts were expressed with >0 or <1 FPKM. The abundant mRNA transcripts of crossbred bull sperm were PRM1 and HMGB4. Gene ontology of transcripts with>1 FPKM revealed there was a major involvement in the structural constituent of ribosomes and translation. Results from pathway enrichment indicated the connection between ribosome, oxidative phosphorylation and spliceosome pathways and the transcripts of crossbred bull spermatozoa. The transcriptional abundance of selected genes, validated using RT-qPCR, indicated significant variations between bulls. Collectively, it may be inferred that the transcripts in crossbred bull sperm were heavily implicated in functions such as the structural constituent of ribosomes and translation, and pathways such as ribosome, oxidative phosphorylation and spliceosome. Further studies using larger sample sizes are required to understand the possible implications of transcriptomic variations on semen quality and fertility.